Electromagnetic acoustic transducers

ABSTRACT

An electromagnetic acoustic transducer for exciting ultrasound in a ferromagnetic material under test ( 2 ), comprises magnetic means ( 6 ) arranged to be moved relative to the material under test ( 2 ) to magnetise a surface layer of the material, and an electrical winding ( 8 ) supplied by an alternating current source, the magnetic means ( 6 ) and the electric winding ( 8 ), in use, being applied in sequence to the material under test ( 2 ) whereby the electrical winding ( 8 ) is positioned adjacent the material subsequent to magnetisation thereof by the magnetic means ( 8 ), the alternating magnetic flux created by the winding ( 8 ) interacting with the remanent magnetisation of the material to create ultrasonic vibration of the material.

TECHNICAL FIELD

This invention relates to electromagnetic acoustic transducers forinspecting ferromagnetic materials, particularly though not exclusivelygas pipelines.

BACKGROUND ART

Non-destructive inspection of metallic materials, for example materialsfrom which structural engineering components are composed, can beundertaken by introducing ultrasound into the material. Informationabout defects within the material can then be obtained by receiving andanalysing the ultrasound signal after it has travelled within thematerial.

Ultrasound is commonly introduced into a material by means ofpiezoelectric transducers, which have a face that vibrates mechanicallyat ultrasound frequencies. The ultrasound is normally passed into thematerial to be tested by a coupling medium, for example water, which isintroduced between the transducer face and the material under test.

For the ultrasonic testing of some materials, it is impractical, or insome cases disadvantageous, to introduce a coupling medium, for exampleduring the inspection of gas pipelines by a moving inspection vehicle,commonly referred to as an inspection pig. Under these circumstances ameans of dry-coupling the ultrasound into the material is required.

Ultrasound can be excited in electrically conductive materials by theapplication of a high frequency magnetic field in the presence of asecond magnetic field which is permanent or varies very slowly. Devicesfor achieving this are called electromagnetic acoustic transducers(hereinafter referred to as EMATs).

For EMATs used in conjunction with a permanent magnetic field, the usualmeans of operation is that the high frequency magnetic field created bythe EMAT produces electrical eddy currents within the material. Theseeddy currents flow in the presence of the permanent magnetic field andgenerate lorentz forces acting within the material. These forces createmechanical displacements within the material, which propagate asacoustic waves. In the ferromagnetic material there are two othermechanisms by which the high frequency magnetic field initiates acousticwaves, namely magnetostriction and magnetic body forces. Theseadditional forces can also play a part in the initiation of acousticwaves.

Numerous designs for EMATs have been proposed each of which ischaracterised by a particular geometry for the two key components of theEMAT, namely the mechanism for generating a permanent magnetic field inthe test specimen and the electrical winding used to carry the highfrequency electrical current. The result of altering these components ofthe EMAT can be that the ultrasound introduced into the test specimen isaltered in the direction it is radiated or in the propagation mode, forexample compression mode or transverse (shear) mode.

Nearly all EMAT transducers suffer from ‘barkhausen’ noise when movedover the surface of a ferromagnetic system. Barkhausen noise is due tothe discontinuous motion of ferromagnetic domain boundaries duringchanges in the bulk magnetisation of ferromagnetic material below thesaturating field for the material. It is a consequence of themagnetising components within the EMAT transducer. Barkhausen noise canbe a severe problem for EMAT pipe inspection using pigs, because theinspection is carried out at speed.

There are a variety of compromises made in the design of any EMAT, butthe arrangement of the magnetic field components and the electricalwindings can only be manipulated within limits set by the fundamentalphysical method of their operation. Each type of EMAT has itsfundamental arrangement of magnets and windings. Once this has beenselected, usually so as to create a desired wave mode, other aspects ofthe design, such as size, thickness of wear plate, mass, rigidity, heattolerance and power handling, can be decided. These are practicalchoices that are constrained by the operating environment, and oftenlimit the acoustic performance of the transducer as measured by, forexample, the acoustic output amplitude or the prominence of thebarkhausen noise. The design problem is therefore strongly affected bythe fundamental method of the EMAT operation, which in some environmentsmay preclude any practical solution.

SUMMARY OF THE INVENTION

It would be desirable to be able to provide an EMAT the construction ofwhich is such as to provide significantly more flexibility in the designof the transducer, and which allows the transducer to be used in a widerrange of operating environments than heretofore.

According to one aspect of the present invention there is provided anelectromagnetic acoustic transducer for exciting ultrasound in aferromagnetic material under test, the transducer comprising magneticmeans arranged to be moved relative to the material under test tomagnetise a surface layer of the material, and an electrical windingsupplied by an alternating current source, the winding being arranged tobe positioned adjacent the material subsequent to magnetisation thereofby the magnetic means, whereby the alternating magnetic flux created bythe winding interacts with the remanent magnetisation of the material tocreate ultrasonic vibration of the material.

It will be appreciated that, with such an arrangement, the magneticcomponent of the transducer is physically separate from the electricalwinding component, the two components of the transducer being applied insequence to the material under test with an arbitrary time intervalbetween the application of the two components.

Such separation of the two components has a very large effect on thepossibilities of transducer design. In some specific applications, forexample pipe inspection, a variety of technical and commercialadvantages are achieved. Furthermore, barkhausen noise is eliminated.

The magnetic means, which may contain permanent magnet materials or anelectromagnetic yoke, may be drawn linearly over the surface of thematerial under test or may be rolled over the surface. In each case thepattern of remanent magnetisation takes the form of one or more stripsof differently orientated remanent magnetisation each following thetrajectory of the magnetic means.

The continuous electrical winding may comprise one or moreinterconnected coils, which may be wound in a plane, for example as apancake or meander coil, or wound around a ferromagnetic core, such as aC-core. In a preferred embodiment of the invention, the magnetic meansand the electrical winding are linked together to be movable as a singleassembly (albeit with some degree of flexibility to allow each toconform to the test surface), the electrical winding following the pathof the magnetic means with a predetermined separation therebetween.

According to a further aspect of the present invention there is provideda method of exciting ultrasound in a ferromagnetic test materialcomprising the steps of establishing a pattern of remanent magnetisationin a surface layer of the material, and, subsequent to saidmagnetisation, applying alternating magnetic flux to the material tointeract with the remanent magnetic field thereby to create ultrasonicvibration of the material.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows, partly cut-away, an EMAT according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to the drawing, a ferromagnetic material under test, which maybe, for example, a high pressure steel gas pipeline, is indicatedgenerally at 2, and an EMAT according to the invention is indicatedgenerally at 4 for generating horizontally polarised guided shear waves.

The EMAT comprises two distinct and separate components, namely amagnetiser indicated generally at 6 and an electrical winding assemblyindicated generally at 8.

The magnetiser 6 includes a linear array of magnets 10 with alternatingmagnetic poles N,S the centres of which are spaced apart by a distanceequal to or shorter than half the wavelength of the desired ultrasoundto be established in the material of the pipeline 2. The magnets 10 areshrouded by a band of wear resistant non-magnetic material shown partlycut away at 12 which does not interfere with or cover the underside ofthe magnets 10 but serves to limit the abrasion of the magnets 10against the test material 2—the underside of the magnets 10 may makecontact with the pipeline 2 or may be spaced therefrom by a very smallgap.

The magnetiser 6 includes a housing 14 containing the magnets 10 andwhich allows convenient attachment of the magnetiser 6 by a firstlinkage 16 to a supporting structure 18 whereby the magnetiser 6 can bepulled along the test surface in a direction perpendicular to that ofthe array of magnets 10.

The electrical winding assembly 8 comprises a set of C-cores 20 thecoils of which are interconnected with one another to form a singlecontinuous winding 22. The cores 20 are mounted on a wear plate 24 ofelectrically insulating material adapted to engage the surface of thepipeline 2. The plate 24, Which may be several millimetres thick whilsthaving negligible effect on the acoustic efficiency of the device whenoperating at frequencies useful for pipe inspection, protects the cores20 from the material surface.

The assembly 8 includes a housing 26 which screens the cores 20 againstelectromagnetic interference, which is important when receiving acousticsignals, and which enables the assembly 8 to be readily connected to thesupporting structure 18 by means of a second linkage 28. The assembly 8is thus positioned rearwards of the magnetiser 6 with the cores 20 in arow perpendicular to the anticipated direction of travel and parallel tothe row of magnets 10, the coils being positioned so that the magneticflux produced by high frequency excitation of the cores 20 interactswith the surface of the test material 2.

In use, the magnetiser 6 and electrical winding assembly 8 are movedtogether by way of the linkages 16, 28 and the supporting structure 18along the surface of the pipeline 2 such that the cores 20 follow themagnets 10 and with the winding 22 driven by a high frequency electricalsource.

The magnets 10 magnetise the surface of the pipeline 2 as detailed aboveto establish remanent magnetisation therein, the subsequently appliedhigh frequency alternating magnetic flux created by the assembly 8interacting with this remanent magnetisation to initiate horizontallypolarised shear waves within the pipeline 2 from the vicinity of thecores 20, the resultant ultrasound propagating substantially parallelwith the row of cores 20.

If the assembly 8 is used as a receiver, the winding is sensitive tohorizontal shear waves arriving from a direction substantially parallelwith the row of cores 20.

Thus the EMAT of the invention effectively comprises two distinctcomponents which may or may not be linked together mechanically. Thefirst component is a magnetiser containing permanent magnet materials oran electromagnetic yoke which, when placed on or near the surface of aferromagnetic test material and subsequently moved away, introduces apredefined pattern of remanent magnetism in a surface layer of the testmaterial. The remanent pattern can be generated simply by dragging thedevice across the surface in a linear motion. In this case the remanentpattern takes the form of one or more linear strips of differentlyoriented remanent magnetisation, each strip following the trajectory ofthe magnetiser.

In an alternative embodiment, the magnetiser may comprise a cylinderincorporating a magnetic pattern and which can be rolled along the testmaterial to create the remanent magnetisation which again may be in'theform of one or more linear strips of differently orientated remanentmagnetisation, each strip following the line of an associated magneticportion of the cylinder.

The second component is an electrical winding assembly which, whenplaced close to the test material, is capable of generating a highfrequency alternating magnetic flux in the surface of the material. Awide range of windings is possible, and some of these can be combinedwith high frequency electrical yokes. The winding arrangements can bethose conventional to EMAT transducers with the exception that they haveno fixed magnet arrangement associated with them. The winding componentis arranged so that, when the high frequency alternating magnetic fluxinteracts with the pattern of remanent magnetisation, ultrasonicvibrations arise within the test material and propagate through it so asto allow ultrasonic inspection of the material. As an alternative to theillustrated C-cores, the electrical winding assembly may comprise one ormore interconnected flat coils of ‘pancake’ construction.

The generation of ultrasound in the ferromagnetic test material isfundamentally achieved by a two-stage process. The first is thepre-conditioning of the ferromagnetic material, analogous to recording amagnetic pattern onto magnetic media such as magnetic tape. For anymaterial having a significant remanence, such as structural steels, thispattern remains as a stable magnetic configuration in the test materialafter the passing or removal of the magnetiser. The second step is theintroduction of the high frequency alternating magnetic flux, whichinteracts with the remanent magnetic pattern and initiates theultrasound. If the ultrasound is to be received, the material ismagnetically pre-conditioned as before, but the electrical winding isoperated as a reception coil and converts the ultrasound into anelectrical signal.

The described and illustrated arrangement shows the winding following inthe path of the magnetising component, with a convenient separationbetween then, each dragged in linear fashion by a framework thatsupports both. This arrangement is particularly useful on pipelineinspection vehicles, and is suitable for both transmission and receptionof ultrasound.

An important advantage of this type of EMAT is that, when operating as areceiver, that is with the winding acting as a listening device only,the EMAT is free from barkhausen noise. This is because the remanentmagnetic pattern within the ferromagnetic material is stable, that isnot evolving with time, within the frame of reference of the testmaterial at the location of the receiver. This is true irrespective ofwhether the receiver is moving or not. This is different from aconventional receiver which would initiate barkhausen noise during itsmotion over the surface of the test material because the magnetscontained in the receiver continuously modify the magnetisation state ofthe material.

Another important advantage of this type of EMAT is that the practicalengineering of the two key components can be achieved independently. Itis no longer necessary to accommodate the magnets around the windings orvice versa, since they no longer occupy the same physical region. Bothcomponents can have less mass than the conventional combined arrangementof windings and magnets, which improve the dynamics of the system. Themagnetic clamping forces between the transducer and the test materialare confined to the magnetiser, and hence the wear problems occurprincipally on only one unit. The wear problem can then be solved byunconventional means, for example by allowing significant wear to occurand using disposable yoke faces rather than hardened wear surfaces. Inaddition, the thickness of the wear plate or shoe used with theelectrical winding can be much greater than normally used by an EMAT.This is because the winding is highly tolerant to ‘lift-off’, sincetheremanent field is impressed into the test material and does notdiminish with lift-off. This contrasts with a normal EMAT, where thesource of field lifts from the surface along with the winding, and theefficiency reduces very rapidly with lift-off.

The EMAT of the invention compares extremely favourably with existingEMATS for generating horizontally polarised shear waves which use anarray of magnets in close proximity to each other. In such known cases,the applied field at the surface of the plate changes rapidly inamplitude and direction for small changes in spatial position within theplate material immediately underneath the transducer. The complexity ofthe field pattern makes these transducers particularly susceptible tomotion-induced barkhausen noise. Additionally, existing EMATs foroperating in pipeline environments are bulky, suffer from acute loss ofefficiency with sensor lift-off, and are severely effected by abrasion.

1. An electromagnetic acoustic transducer for exciting ultrasound in aferromagnetic material under test (2), the transducer comprisingmagnetic means (6) comprising a linear array of magnets with alternatingmagnetic poles, arranged to be moved relative to the material under test(2) to magnetise a surface layer of the material, and an electricalwinding (8) supplied by an alternating current source; characterised inthat: the centres of said alternating magnetic poles are spaced apart bya distance equal to or shorter than half the wavelength of saidultrasound, and the electrical winding (8) is spaced from the array ofmagnets, whereby, in use, the magnetic means (6) and the electricwinding (8) are arranged to be applied in sequence to the material undertest (2) whereby the electrical winding (8 is for positioning adjacentthe material subsequent to magnetisation thereof by the magnetic means(8), the alternating magnetic flux created by the winding (8) beingarranged to interact with the remanent magnetisation of the material tocreate ultrasonic vibration of the material.
 2. A transducer as claimedin claim 1 in which the magnetic means (6) contains permanent magnetmaterials (10) or an electromagnetic yoke.
 3. A transducer as claimed inclaim 1 or claim 2 in which the continuous electrical winding (8)comprises one or more interconnected coils.
 4. A transducer as claimedin claim 3 in which the coils are wound in a plane, for example as apancake or meander coil, or wound around a ferromagnetic core, such as aC-core (20).
 5. A transducer as claimed in any one of claims 1 to 4 inwhich the magnetic means (6) and the electrical winding (8) are linkedtogether to be movable as a single assembly, the electrical winding (8)following the path of the magnetic means (6) with a predeterminedseparation therebetween.
 6. A method of exciting ultrasound in aferromagnetic test material (2) comprising: pre-conditioning a surfacelayer of the ferromagnetic material, by applying a magnetic fieldthereto using a magnetiser, removing said magnetiser or causing saidmagnetiser to pass from said surface layer, thereby establishing apattern of remanent magnetisation in the surface layer of the material;and subsequently applying alternating magnetic flux to the material tointeract with the remanent magnetic field thereby to create ultrasonicvibration of the material.
 7. A method as claimed in claim 6 using anelectromagnetic acoustic transducer as claimed in any one of claims 1 to5.
 8. A method as claimed in claim 7 in which the magnetic means (6) isdrawn linearly over the surface of the material under test (2).
 9. Amethod as claimed in claim 7 in which the magnetic means (6) is rolledover the surface of the material under test.